An X-ray CT apparatus is an image diagnosis apparatus that irradiates, mainly, radiates an X-ray to detect transmission thereof through a subject and reconstructs an image of the inside of the subject from projection data indicating the intensity of the radiation having been detected. Such an X-ray CT apparatus has rapidly become popular on the frontiers of medicine and, as an indispensable diagnosis apparatus to medical practice, plays an important role in many medical practices including diagnosis of disease and plan of treatment and operation. Also for industrial X-ray inspection apparatuses, CT is commercialized by a like principle. In principle, by utilizing the X-ray's property of passing through a subject, X-ray CT apparatuses radiate an X-ray to a subject from multiple directions, measure projection data by a detector arranged on the opposite side across the subject, and reconstruct an image as a cross-sectional image.
To be specific, a medical X-ray CT apparatus executes slice projection by rotating an X-ray tube serving as an X-ray generation source and a detector arranged on the opposite side thereto across a subject at high speeds while sliding the patient, and measures X-ray projection data to compose a cross-sectional image. Such a medical X-ray CT apparatus has been required to reduce the amount of an X-ray administered to a patient, increase an imaging speed, and increase resolution. For this purpose, the medical X-ray CT apparatuses are provided with a diagnostic technique that realizes high speed and high resolution by a multi-slice technique, an ultrahigh-speed image processing technique, and so on. Moreover, industrial X-ray CT apparatuses generally employ a method of, with an X-ray generator and a detector arranged on the opposite side thereto across a subject fixed, rotating the subject to measure projection data and composing a cross-sectional image.
An X-ray tube serving as an X-ray generation source of an X-ray CT apparatus has an electronic optical system that generates an electron beam, and is configured to make the electron beam emitted from the electronic optical system collide with an anode that rotates high speeds and to generate an X-ray by anode collision. The X-ray generator in the X-ray CT apparatus is composed of the X-ray tube that generates an X-ray by the aforementioned configuration. In the following description: in a case that a coil filament is used as an electronic optical system, a longitudinal direction of the filament shall be an X-axis direction, a travelling direction of an electron beam emitted from the electronic optical system of the X-ray tube shall be a Z-axis direction, and a direction orthogonal to both the X-axis and the Z-axis shall be a Y-direction.
In recent years, in accordance with improvement of X-ray CT apparatuses, it has been desired to develop a next-generation CT scanner that responds to increase of resolution, reduction of pseudo defects and increase of a diagnosis function, by adding a function of varying and moving a focal point to the X-ray generation source (the X-ray tube) used with a fixed focal point. A real focal point of an X-ray CT apparatus refers to a place at which electrons collide with an anode and an X-ray is generated, and coincides with a radiation range of an electron beam on the anode.
An X-ray tube of an X-ray CT apparatus is generally used with a fixed focal point. By slightly moving an electron beam (an X-ray beam) reaching an anode in the orthogonal directions to the body axis of a subject laid on a detection system (the X-axis/Y-axis directions), it is possible to increase spatial resolution of projection data obtained by detecting the radiated X-ray.